## Two Theories of Relativity

**Trending PhD Research Topics:** First, theory of relativity encompasses two interrelated theories by Albert Einstein: special relativity and general relativity. As a matter of fact, Special relativity applies to all physical phenomena in the absence of gravity. And General relativity explains the law of gravitation and its relation to other forces of nature. It applies to the cosmological and astrophysical realm, including astronomy. This conceptual studies also triggered many researchers of these days making it one among the *Trending PhD Research Topics*.

## Pictorial View of Einstein’s Theory of Relativity (Image Courtesy: NASA)

Uniquely, the theory transformed theoretical physics and astronomy during the 20th century, superseding a 200-year-old theory of mechanics. It created primarily by Isaac Newton. Especially, it introduced concepts including space-time as a unified entity of space and time, relativity of simultaneity, kinematic and gravitational time dilation, and length contraction. In the field of physics, relativity improved the science of elementary particles and their fundamental interaction with ushering in the nuclear age. With relativity, cosmology and astrophysics predicted extraordinary astronomical phenomena such as neutron stars, black holes, and gravitational waves.

## Einstein’s Theory of General Relativity

In 1905, Albert Einstein determined that the laws of physics are the same for all non-accelerating observers, and that the speed of light in a vacuum is independent of the motion of all observers. This is the theory of special relativity. It introduce a new framework for all of physics and proposed new concepts of space and time.

Einstein then spent 10 years trying to include acceleration in the theory and published his theory of general relativity in 1915. In it, he determine that massive objects cause a distortion in space-time, which is felt as gravity.

## Einstein’s Theory of Special Relativity

The theory of special relativity explains how space and time are link for objects that are moving at a consistent speed in a straight line. One of its most famous aspects concerns objects moving at the speed of light.

As a matter of fact, as an object approaches the speed of light, its mass becomes infinite and it is unable to go any faster than light travels. This cosmic speed limit has been a subject of much discussion in physics, and even in science fiction, as people think about how to travel across vast distances.

The theory of special relativity was develop through the Albert Einstein in 1905, and it forms part of the basis of modern physics. After finishing his work in special relativity, Einstein spent a decade pondering what would happen if one introduced acceleration. This formed the basis of his general relativity, published in 1915.

## Evolutional Concept Behind Theory of Relativity – PhD Trending Research Topics

Before Einstein, astronomers (for the most part) understood the universe in terms of three laws of motion present through Isaac Newton in 1686. These three laws are:

(1) Objects in motion (or at rest) remain in motion (or at rest) unless an external force imposes change.

(2) Force is equal to the change in momentum per change of time. For a constant mass, force equals mass times acceleration.

(3) For every action, there is an equal and opposite reaction.

## Special Relativity and Quantum Mechanics

Likewise, as our knowledge of physics has advanced, scientists have run into more counter intuitive situations. One is trying to reconcile general relativity — which describes well what’s going on with large objects — with quantum mechanics, which is best used for very small things (such as uranium atom decay). The two fields, which excellently describe their individual fields, are incompatible with one another — which frustrated Einstein and generations of scientists after him.

“Relativity gives nonsensical answers when you try to scale it down to quantum size. Eventually descending to infinite values in its description of gravity. Likewise, quantum mechanics runs into serious trouble when you blow it up to cosmic dimensions,” an article in The Guardian pointed out in 2015.

## Executive Summary

But there were cracks in the theory for decades before Einstein’s arrival on the scene, according to Encyclopedia Britannica. In 1865, Scottish physicist James Clerk Maxwell demonstrated that light is a wave with both electrical and magnetic components. Established the speed of light (186,000 miles per second). Scientists supposed that the light had to be transmitted through some medium, which they called the ether. (We now know that no transmission medium is required, and that light in space moves in a vacuum.)

Twenty years later, an unexpected result threw this into question. Physicist A.A. Michelson and chemist Edward Morley (both Americans at the time) calculated how Earth’s motion through this “ether” affected how the speed of light is measured. Also, found that the speed of light is the same no matter what Earth’s motion is. This led to further musings on light’s behavior — and its incongruence with classical mechanics — by Austrian physicist Ernst Mach and French mathematician Henri Poincare.

Einstein began thinking of light’s behavior when he was just 16 years old, in 1895. He did a thought experiment. The encyclopedia said, where he rode on one light wave and look at another light wave moving parallel to him.

## Conclusion

Classical physics should say that the light wave Einstein was looking at would have a relative speed of zero. This contradict Maxwell’s equations that show light always has the same speed: 186,000 miles a second. Another problem with relative speeds is they show that the laws of electromagnetism change depending on your vantage point. Which contradicted classical physics as well (which said the laws of physics were the same for everyone.)

This led to Einstein’s eventual musings on the theory of special relativity, which he broke down into the everyday example of a person standing beside a moving train. Comparing observations with a person inside the train. He imagined the train being at a point in the track equally between two trees. If a bolt of lightning hit both trees at the same time, due to the motion of the train, the person on the train would see the bolt hit one tree before the other tree. But the person beside the track would see simultaneous strikes.

“Einstein conclude that simultaneity is relative; events that are simultaneous for one observer may not be for another,” the encyclopedia stated. “This led him to the counter intuitive idea that time flows differently according to the state of motion, and to the conclusion that distance is also relative.”

**References**

**[1]** https://en.wikipedia.org/wiki/Theory_of_relativity

**[2]** https://www.space.com/17661-theory-general-relativity.html

**[3]** https://www.space.com/36273-theory-special-relativity.html

**Written by,**

**MeenatchiSunndhar P I, M.E., AMIE.**

**(With BEC Vantage (B2 in CEFR) – IELTS band score of 5.5 and Cambridge English Scale Score: 160)**